# Crystal structures

Entry by Emily Redston, AP 225, Fall 2011

Crystalline materials are characterized by repeating (or periodic) arrays of atoms over large atomic distances. Typically we think about these materials in the context of solids, though there are also liquid crystals. Crystallization is the process by which crystal structures form; atoms will position themselves in a repetitive three-dimensional structure upon solidification. The system is said to display long-range order. If a solid does not have long-range order, it is considered amorphous. The **crystal structure** of a material is the unique way in which atoms, ions, or molecules are spatially arranged. There is an extremely large number of different crystal structures that all have long-range order. Some of these are relatively simple structures (e.g. metals) while others can be exceedingly complex (e.g. ceramic and polymeric materials). When describing crystal structures, it is useful to think about the atoms as solid spheres having well-defined radii. We call this the atomic hard sphere model, where spheres representing nearest neighbor atoms touch one another. The term lattice is used to describe a three-dimensional array of points that coincide with atom positions.

In describing crystal structures, it is often convenient to sub-divide the structure into small repeat entities called unit cells. We typically choose a unit cell such that it represents the symmetry of the crystal structure. The unit cell is a small structure that repeats itself by translation through the crystal; if the unit cells are stacked together in three-dimensions, they describe the bulk arrangement of atoms of the crystal. Thus the unit cell is the basis structural unit of the crystal structure, and it defines the crystal structure by its geometry and the atom positions within. The unit cell is described by its lattice parameters, which are the length(s) of the cell edges and the angles between them.

Many crystal structures are based on a close-packing of atoms. Since they are relevant to hard sphere colloidal packing, I will go over two essential structures: face-centered cubic and hexagonal close packed.

For hard-sphere models, you typically end up with relatively large numbers of nearest neighbors and dense atomic packings due to the minimal restrictions as to the number and position of nearest-neighbor atoms.

The face-centered cubic system (F) has lattice points on the faces of the cube, that each gives exactly one half contribution, in addition to the corner lattice points, giving a total of 4 lattice points per unit cell (1⁄8 × 8 from the corners plus 1⁄2 × 6 from the faces).

There are two simple regular lattices that achieve this highest average density. They are called face-centered cubic (fcc) (also called cubic close packed) and hexagonal close-packed (hcp), based on their symmetry. Both are based upon sheets of spheres arranged at the vertices of a triangular tiling; they differ in how the sheets are stacked upon one another.

In both the fcc and hcp arrangements each sphere has twelve neighbors. For every sphere there is one gap surrounded by six spheres (octahedral) and two smaller gaps surrounded by four spheres (tetrahedral). The distances to the centers of these gaps from the centers of the surrounding spheres is \scriptstyle \sqrt{\frac{3}{2}} for the tetrahedral, and \scriptstyle \sqrt2 for the octahedral, when the sphere radius is 1.

Relative to a reference layer with positioning A, two more positionings B and C are possible. Every sequence of A, B, and C without immediate repetition of the same one is possible and gives an equally dense packing for spheres of a given radius.

The most regular ones are:

fcc = ABCABCA (every third layer is the same) hcp = ABABABA (every other layer is the same)

In close-packing, the center-to-center spacing of spheres in the x–y plane is a simple honeycomb-like tessellation with a pitch (distance between sphere centers) of one sphere diameter. The distance between sphere centers, projected on the z (vertical) axis, is:

\text{pitch}_Z = \sqrt{6} \cdot {d\over 3}\approx0.81649658 d,

where d is the diameter of a sphere; this follows from the tetrahedral arrangement of close-packed spheres.

The coordination number of hcp and fcc is 12 and its atomic packing factor (APF) is the number mentioned above, 0.74.

## References

[1] Callister, William D. *Materials Science and Engineering: an Introduction*. New York: John Wiley & Sons, 2007.

[2] http://en.wikipedia.org/wiki/Close-packing_of_equal_spheres

## Keyword in references:

Phase Behavior and Structure of a New Colloidal Model System of Bowl-Shaped Particles